Project summary/abstract Dendritic cells play an important role in inducing antigen-specific immune responses through the ability to present antigens to nave T cells and differentiate them to specific effector cells. This function is profoundly dependent on proper trafficking of membrane proteins, but the molecular mechanisms that govern the specific trafficking is not completely understood. Our laboratory is interested in characterizing intracellular trafficking of membrane proteins playing essential role in dendritic cell function. Our current research is focused on understanding the mechanism and function of the membrane trafficking mediated by a ubiquitin ligase named membrane associated RIGN-CH1 (MARCH1). MARCH1 attaches a ubiquitin chain to the cytoplasmic tail of MHCII and CD86, and this ubiquitination induces endocytosis, lysosomal sorting, and degradation of the molecules. We have previously found that MARCH1-dependent MHCII ubiquitination is required for dendritic cell function of selecting regulatory T cells. The mechanisms involved the ubiquitin ligase activity of MARCH1 in maintaining MHCII proteostasis in the plasma membrane of dendritic cells, which was crucial for dendritic cells to stably engage and activate immature thymocytes for regulatory T cell differentiation. We will investigate whether this activity also supports dendritic cell function of inducing antigen-specific T cell immunity. In addition, we will identify new substrates of MARCH1 in dendritic cells and interrogate the molecular interactions involved in MARCH1?s ubiquitin ligase activity. The information gained from these studies will not only provide important new insights into the role of MARCH1-dependent membrane trafficking in dendritic cell function but also improve our understanding on the mechanisms of membrane protein ubiquitination.